1. Field of the Invention
The present invention is directed to control signaling in communication systems and, more specifically, to virtual length extension of a Cyclic Redundancy Check (CRC) by utilizing existing fields in control signaling, when the existing fields do not need to convey their intended information. The present invention is further considered in the development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE).
2. Description of the Art
A User Equipment (UE), also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A base station (Node B) is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
The DownLink (DL) signals include data signals, control signals, and reference signals (also referred to as pilot signals). The data signals carry the information content and can be sent from the Node B to UEs through a Physical Downlink Shared CHannel (PDSCH). The control signals may be of broadcast or UE-specific nature. Broadcast control signals convey system information to all UEs. UE-specific control signals include, among others, the Scheduling Assignments (SAs) for DL data packet reception or UpLink (UL) data packet transmission and are part of the Physical Downlink Control CHannel (PDCCH). The Reference Signals (RS) serve multiple UE functions including channel estimation for PDSCH or PDCCH demodulation, measurements for cell search and handover, and Channel Quality Indication (CQI) measurements for link adaptation and channel-dependent scheduling.
The DL and UL data packet transmission (or reception) time unit is assumed to be a sub-frame.
A DL sub-frame structure is illustrated in FIG. 1 and corresponds to one of the structures used in the 3GPP E-UTRA LTE.
Referring to FIG. 1, the sub-frame includes 14 OFDM symbols 110. Each OFDM symbol is transmitted over an operating BandWidth (BW) including OFDM sub-carriers 120.
Further, 4 Node B transmitter antenna ports are assumed. The RS from antenna port 1, antenna port 2, antenna port 3, and antenna port 4 is respectively denoted as RS1 130, RS2 140, RS3 150, and RS4 160. The PDCCH and PDSCH multiplexing is in the time domain, with the PDCCH 170 occupying at most the first N OFDM symbols. At least the remaining 14-N ones are typically assigned to PDSCH transmission 180, but may occasionally contain transmission of synchronization and broadcast channels.
An OFDM transmitter is illustrated in FIG. 2.
Referring to FIG. 2, the OFDM transmitter includes a Coder and Interleaver 220, a Modulator 230, a Serial to Parallel (SIP) Converter 240, an Inverse Fast Fourier Transformer (IFFT) 250, and a Parallel to Serial (P/S) Converter 260. Information data 210 is first encoded and interleaved in Coder and Interleaver 220. For example, the coding may be performed using turbo coding (with a given redundancy version) and the interleaving may be performed using block interleaving. The encoded and interleaved data is then modulated by the Modulator 230, for example, using QPSK, QAM 16, or QAM64 modulation. A serial to parallel conversion is then applied by the S/P Converter 240 to generate M modulation symbols, which are subsequently provided to the IFFT, which effectively produces a time superposition of M orthogonal narrowband sub-carriers. The M-point time domain blocks obtained from the IFFT 250 are then serialized by the P/S Converter 260 to create a time domain signal 270. The RS transmission can be viewed as non-modulated data transmission. Additional functionalities, such as data scrambling, cyclic prefix insertion, time windowing, filtering, etc., are well known in the art and are omitted for brevity.
The reverse functions are performed at an OFDM receiver as illustrated in FIG. 3.
Referring to FIG. 3, the OFDM receiver includes an SIP Converter 320, a Fast Fourier Transformer (FFT) 330, a P/S Converter 340, a Demodulator 350, and a Decoder and Deinterleaver 360. The received signal 310 is provided to the S/P Converter 320 to generate M received signal samples, which are then provided to the FFT 330. After the output of the FFT 330 is serialized by the P/S Converter 340, it is provided to the Demodulator 350 and then to the Decoder and Deinterleaver 360. Finally, the decoded data is obtained and Cyclic Redundancy Check (CRC) evaluation is performed to determine correct or incorrect reception.
Similarly to the OFDM transmitter illustrated in FIG. 2, well known in the art functionalities such as filtering, time-windowing, cyclic prefix removal, de-scrambling, channel estimation, etc., using the RS, are not shown for brevity.
An operating BW is divided into elementary scheduling units, referred to as Physical Resource Blocks (PRBs). For example, a PRB may include 12 consecutive OFDM sub-carriers. This allows the Node B to configure, through the PDCCH, multiple UEs to transmit or receive data packets in the UL or DL, respectively, by assigning different PRBs for packet transmission or reception from or to each UE, respectively. For the DL, this concept is illustrated in FIG. 4.
FIG. 4 is a diagram illustrating a scheduling of data packet transmissions to UEs in PRBs over one sub-frame.
Referring to FIG. 4, 5 out of 7 UEs are scheduled to receive PDSCH in one sub-frame 410 over 8 PRBs 420. UE1 430, UE2 440, UE4 450, UE5 460, and UE7 470, are scheduled PDSCH reception in one or more PRBs, while UE3 480 and UE6 490 are not scheduled any PDSCH reception. The allocation of PRBs may or may not be contiguous and a UE may be allocated an arbitrary number of PRBs (up to a maximum as determined by the operating BW and the PRB size).
FIG. 5 is a block diagram illustrating a coding process of an SA at the Node B. For the description of FIG. 5, it is assumed that the Node B separately encodes all SAs. The Medium Access Control (MAC) layer UE IDentity (UE ID), for the UE an SA is intended for, masks the CRC of the SA codeword in order to enable the reference UE to identify that the particular SA is intended for it.
Referring to FIG. 5, the CRC 520 of the (non-coded) SA bits 510 is computed and is subsequently masked 530 using an exclusive OR (XOR) operation between CRC bits and the MAC layer UE ID 540, where XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC is then appended to the SA bits 550, channel coding is performed 560, for example, using a convolutional code, followed by rate matching 570 to the allocated PDCCH resources. interleaving and modulating 580, and transmission of the control signal 590 conveying the SA.
FIG. 6 is a block diagram illustrating a decoding process of an SA at the UE.
Referring to FIG. 6, the UE receiver performs the reverse operations of the Node B transmitter to determine whether the UE has an SA in a sub-frame. More specifically, the received control signal 610 is demodulated and de-interleaved 620. Rate matching applied in the Node B transmitter is restored at the UE by the rate matcher 630, and the data is subsequently decoded 640.
After decoding, the SA bits 660 are obtained, after extracting the CRC bits 650, which are then de-masked 670 by applying the XOR operation with the UE ID 680. Finally, the UE performs the CRC test 690. If the CRC test passes, the UE determines that the SA is valid and determines the parameters for signal reception (i.e., DL SA) or signal transmission (i.e., UL SA). However, if the CRC test does not pass, the UE disregards the SA.
Information Elements (IEs) in a DL SA and a UL SA are provided in Table 1 and are consistent with the ones used in 3GPP E-UTRA LTE. It is assumed herein that both the CRC and the UE ID consist of 16 bits.
TABLE 1Information Elements in DL SA and UL SANumberIE for ULNumberIE for DL SAof BitsSAof BitsCommentFlag1Flag1To distinguish UL SA from DL SA (e.g. 0for UL SA, 1 for DL SA)PRB11PRB11Specified by ceil(log2(NPRB(NPRB + 1)/2)) bitsAllocationAllocation(NPRB = 50 is assumed)MCS5MCS5Modulation and Coding Scheme (MCS)LevelsHARQ3Cyclic Shift3Hybrid Automatic Repeat reQuest (HARQ)ProcessIndicatorprocess number in DL SA Cyclic Shift(CSI)Indication for RS Transmission in UL SANDI1NDI1New Data IndicatorRV2CQI Request1HARQ Redundancy Version (RV) in DL SAChannel Quality Indicator (CQI)Transmission (yes/no) in UL SATPC2TPC2Transmission Power Control (TPC)commandsCRC (UE ID)16CRC (UE ID)16CRC masked by UE IDTotal Bits41Total Bits411 bit padding (fixed value) for UL SA toDL SAUL SAobtain same size with DL SA
In Table 1, the operating BW is assumed to comprise of 50 PRBs and consecutive PRB assignment is considered as an example. For consecutive allocations over a maximum of NPRB, the total number of combinations is determined as 1+2+ . . . +NPRB=NPRB(NPRB+1)/2 which can be signaled with an IE having ceil(log2 (NPRB(NPRB+1)/2)) bits, where the “ceil” operation rounds a number to its next integer.
The Cyclic Shift Indicator (CSI) IE specifies the cyclic shift applied in the Constant Amplitude Zero Auto-Correlation (CAZAC)-based sequence used to form the RS transmitted by the UE.
HARQ is assumed to apply for the data packets transmissions and the respective information is given by a corresponding IE (for the DL only as the UL HARQ process is assumed to be synchronous).
The New Data Indicator (NDI) IE specifies the beginning of a new Hybrid Automatic Repeat reQuest (HARQ) process and the Redundancy Version (RV) IE corresponds to data packet re-transmissions for the same HARQ process.
The CQI request IE indicates whether the UE should include or not a CQI report with its scheduled UL transmission.
The Modulation and Coding Scheme (MCS) IE specifies a modulation scheme, such as QPSK, QAM16, or QAM64, and a coding rate from a set of possible coding rates, for a predetermined coding method, such as turbo coding.
The Transmission Power Control (TPC) IE is associated with the application of power control for data or control signal transmissions from the reference UE.
One application of particular interest in communication systems is Voice over Internet Protocol (VoIP). Due to the large number of UEs that may typically require VoIP services, it is desirable to not send SAs to UEs in every sub-frame because the associated PDCCH overhead becomes excessively large, which affects the overall efficiency and throughput of the communication system. Accordingly, Semi-Persistent Scheduling (SPS) is used instead.
With SPS, data packet transmissions to or from VoIP UEs are activated once using an SA and subsequent initial packet transmissions continue periodically without new SAs (SAs may still be used for re-transmissions, if the initial transmission is incorrectly received). For the same UE, by using a different MAC UE ID, SPS SAs may be distinguished from SAs for dynamic scheduling, where each data packet transmission is associated with an explicit SA.
However, a consequence of separately encoding the SAs is that a UE then needs to perform multiple decoding operations and CRC tests in order to determine whether it has a valid DL SA or UL SA. Further, for the UEs without any SA, the decoding operations need to exhaust an entire search space in the PDCCH for possible SAs, before eventually determining that no SAs are directed to them. Consequently, this increases the number of decoding operations. For example, even if measures to limit its value are applied, as in EUTRA LTE, at least about 40 decoding operations may be required.
Assuming random PDCCH bits and a 16-bit CRC, a false positive SPS activation (CRC test incorrectly passes) from a UE without an SA occurs, on average, after 216=65536 CRC tests. For a sub-frame duration of 1 millisecond and 40 decoding operations per sub-frame, the average time between false positive SPS activations is 216/40 milliseconds or about 1.64 seconds. Although accounting for discontinuous packet reception for VoIP UEs or for the Voice Activity Factor (VAF) will somewhat increase the average time of consecutive false positive SPS activations, for example by a factor of about 10, this average time will still be in the order of seconds.
If an SPS UE (such as, for example, a VoIP UE) has a false positive SPS activation, the consequences depend on whether the SA is interpreted as a DL one or as a UL one. If a UE incorrectly determines that it has a DL SA, it will fail to decode the presumed data packet transmission from the Node B (because no such data packet exists) and it will periodically transmit a Negative ACKnowledgement (NACK) in the UL of the communication system. This NACK may collide with a NACK or with a positive ACKnowledgement (ACK) transmitted from a UE with valid PDSCH reception. This is problematic when the UE with the valid PDSCH reception transmits an ACK.
A more detrimental operating condition results when an SPS UE incorrectly determines that it has a UL SA. In this case, the UE will be transmitting data in the UL, which will interfere with data transmitted by one or more other UEs with valid SAs. The fundamental consequence of such interference is that the UL communication reliability for affected UEs either with valid SAs or with invalid SAs will be seriously compromised.
Therefore, there is a need to reduce the probability of false positive SPS activations for SPS UEs and respectively increase the time period between two successive CRC tests passing incorrectly.
There is another need to avoid increasing the CRC size in order to avoid increasing the associated overhead.
There is another need to maintain the same size between dynamic SAs and SPS SAs in order to minimize the decoding operations a UE needs to perform, thereby minimizing implementation complexity and power consumption.